This invention relates to the manufacture of powdered/microfibrillated oxidized cellulose suitable for use as an immobilizing matrix or a carrier for drugs, chemicals, and biological macromolecules having applications in the pharmaceutical, medical, veterinary, and agricultural fields. Specifically, this invention relates to a method and means of producing oxidized cellulose in high yields and with different levels of oxidation involving a treatment of a cellulose source with a mixture of phosphoric acid and nitric acid and sodium nitrite.
Oxidized cellulose (OC) containing less than 3% carboxylic content is useful as a direct compression tabletting excipient and as a drug carrier and a bodying agent in the development of topical and transdermal formulations (see Banker and Kumar, U.S. Pat. Nos. 5,414,079 and 5,405,953), whereas OC with xe2x89xa73% carboxylic groups serves as a biodegradable material (see Ashton, U.S. Pat. No. 3,364,200). OC containing 16-24% carboxylic content has been accepted for use in humans to stop bleeding during surgery and to prevent the formation and reformation of post surgical adhesions (1). Studies also show that OC possesses antibacterial activity (2), promotes bone regeneration (3), and is useful in periodontal therapy (4). These properties have been related to the polyglucuronic acid structure of OC.
The carboxyl content and degree of polymerization (DP) of oxidized cellulose have been reported to play important roles in the degradation of oxidized cellulose in-vitro and in-vivo. In general, the higher the carboxyl content, or the lower the DP, the faster the degradation of OC (5). In-vitro solubilization and degradation studies have shown OC to be readily hydrated. About 90% of OC (carboxyl content 12-18%) converts to solubilized substances within 21 days in a pH 7.4 buffer solution. An analysis of the resulting OC solution by high performance liquid chromatography (HPLC) suggested that the polymer readily undergoes chain shortening to yield oligomers (6). In the presence of plasma or serum, the oligomers are further hydrolyzed to small fragments, such as glucose, glucuoronic acid and 2,3-carbon fragments. It has been suggested that the degradation of OC to oligomers occurs due to the presence of the carboxyl group at the C-6 position, which increases the susceptibility of the intersaccharide linkage to hydrolytic cleavage. Dimitrijevzh et al. (7) studied the degradation of the OC (carboxyl content 12-18%) in-vivo. They implanted oxidized cellulose onto rabbit uterine horn abrasions. Degradation was found to be rapid, and the oligomeric products produced were present primarily in the peritoneal fluid at the implantation site. No accumulation was observed in either the serum or urine. A mechanism of degradation consisting of chemical depolymerization, followed by enzymatic hydrolysis mediated by glycosidases endogenous to peritoneal macrophages, was proposed in this study.
Several studies on the use of OC as an immobilization matrix for drugs, and enzymes have been reported (9-20). Dol""berg et al. (9) reported the preparation, characterization and evaluation of ionic complexes of kanamycin sulfate and sulfanilamide with oxidized cellulose fiber. Compared to free drugs, these complexes exhibited biological effects in excess of twenty days, and were readily absorbed in-viuo. Adrenalone complexed with oxidized cellulose showed prolonged antiseptic and anesthetic activities and was useful for treating parodontosis (12,13). Firsov et al. (15) found that the ionic complexes of lincomycin and OC were less irritating to the skin and mucous membranes. Implants of an ionic complex of gentamycin and OC showed antibiotic concentrations at the site of implantation for 30 days (15). Recently, several chemotherapeutic agents, such as photrin (16), dimetpramide (17), and a mixture of methotrexate and hydroxythiamine (18), have been immobilized on OC and were shown to be more effective than the respective free drugs. When trypsin was immobilized on OC, it exhibited higher activity than when it was immobilized on phosphate or amino functionalized cellulose (19). Increased activity was also observed when proteinase, an enzyme, was immobilized on OC (20).
Currently, oxidized cellulose containing 16-24% carboxylic content is commercially available in powder, gauze and fabric forms. However, it is relatively expensive to be used as an excipient in the development of a pharmaceutical product. The existing methods to produce OC typically involve a reaction between cellulose and an oxidant. The latter includes nitrogen dioxide or dinitrogen tetraoxide, dichromate/dinitrogen tetraoxide, nitric acid, nitric acid-sodium nitrite, sulfuric acid-nitric acid-sodium nitrite, phosphoric acid-sodium nitrite, phosphoric acid-sodium nitrite-sodium nitrate, and phosphoric acid nitrogen dioxide, hypohalites, sodium persulfate, etc. (21). Oxidants that produce OC with less than 3% carboxylic content have been disclosed in U.S. Pat. Nos. 5,414,079 and 5,405,953. They are non-specific in their mode of action and often introduce aldehyde and/or ketone groups in addition to the carboxylic groups.
The primary consideration in the preparation of biodegradable oxidized cellulose having 3% or higher carboxylic groups is the uniformity and extent of oxidation. Of the various oxidizing agents investigated, nitrogen dioxide has been the most extensively studied for cellulose. It selectively converts the C-6 primary hydroxyl group to the carboxyl group. Nitrogen dioxide can be used in the gaseous form or as a solution in an appropriate organic solvent.
French patent 2,408,624 (22) and Netherlands patent 7,711,034 (23) disclose the preparation of oxidized cellulose materials suitable for use as a homeostatic agent, by treatment of cotton gauze with a HNO3xe2x80x94NaNO2 mixture for 24 hours. The product contained 14-18% carboxyl group, and reportedly dissolved in 0.1 M NaOH.
In U.S. Pat. No. 2,758,112 (24), a mixture of H2SO4xe2x80x94HNO3xe2x80x94NaNO2 was used to prepare OC from cellulose fibers. This product contained about 16% carboxyl content. Compared to the NHO3xe2x80x94NaNO2 method, this process requires a much shorter reaction time and is more cost effective. The present inventor has found, however, that the yield of OC by this method is very low (xcx9c20%).
Walimbe et al. (25) reported a two-stage oxidation process to produce OC, first using an acid-dichromate mixture and then with nitrogen dioxide as oxidants. The main advantage of this method is that the reaction period of vapor phase oxidation with NO2 is reduced considerably. This method allows the manufacture of OC with 6-18% carboxylic content. However, the reaction conditions are difficult to control. Further, this method introduces chromium, which is objectionable.
Recently, Heinze et al. (26) investigated the oxidation of cellulose under homogeneous conditions by dissolving the starting cellulose in H3PO4 and then oxidizing with N2O3, produced in situ following addition of the NaNO2. Using this approach, they were able to produce OC with an 80% yield and with up to a 21% carboxyl content. However, the initial dissolution step is slow at room temperature and requires a long reaction period. Reportedly, heating the solution to a higher temperature facilitates the dissolution of cellulose but also causes a rapid hydrolysis of cellulose (Wei and Banker, U.S. Pat. No. 5,417,984).
The role of phosphoric acid in the oxidation of cellulose was also studied by Bertocchi et al. (27). They swelled cellulose in 85% H3PO4 for 0-5 hours at 4xc2x0 C. prior to treatment with NO2 gas. The pretreatment of cellulose with H3PO4 produced a good yield and an acceptable oxidation level of OC. However, the DP was too high (xcx9c900) because the phosphoric acid is a relatively weak acid. This limits its use in pharmaceutical application. More recently, Gert et al. (28) prepared OC in a powder form by treating cellulose with NHO3 at elevated temperatures (50-100xc2x0 C.). Both oxidation and hydrolysis (degradation) of cellulose have been reported to occur simultaneously under these conditions.
From the discussion above, it is obvious that there is a need in the art for a method of producing OC in different oxidation levels and high yields. There is also a need in the art for a method of producing finally powdered/microfibrillated, colloidal or microsphere forms of OC so that pharmaceutical applications of OC may be possible.
Accordingly, it is a primary objective of the present invention to provide a method and means of producing oxidized cellulose in high yields and different oxidation levels.
It is a further objective of the present invention to provide a method and means of producing oxidized cellulose suitable for use as a biodegradable carrier or immobilizing matrix for a variety of drugs, proteins, and enzymes.
It is still a further objective of the present invention to provide a method and means of producing oxidized cellulose using a variety of cellulose sources, including cotton linters, purified cotton papers, a-cellulose, purified wood pulp, microcrystalline cellulose, powdered cellulose, or like materials.
It is a further objective of the present invention to provide a method and means of producing oxidized cellulose that is suitable for the manufacture of sustained- and/or controlled release delivery systems for drugs and other chemical agents.
It is still a further objective of the present invention to provide a method and means of producing oxidized cellulose that is suitable for use in stopping bleeding during surgery.
It is yet a further objective of the present invention to provide a method and means of producing oxidized cellulose that is suitable for use in preventing the formation and reformation of surgical adhesions.
It is a further objective of the present invention to provide a method and means of producing oxidized cellulose that is suitable for promoting bone regeneration.
It is a further objective of the present invention to provide a method and means of producing oxidized cellulose that is suitable for use in periodontal therapy.
Yet a further objective of the present invention is to provide a method and means of producing oxidized cellulose that is economical.
The method and means of accomplishing each of the above objectives as well as others will become apparent from the detailed description of the invention which follows hereafter.
The present invention provides a new method of preparing oxidized cellulose with different levels of oxidation and in high yields, suitable for use in pharmaceutical products as a carrier for a variety of drugs, proteins, and enzymes. Oxidized cellulose with less than 3% carboxylic content serves as a non-degradable carrier system, whereas that containing equal to or greater than 3% carboxylic groups is useful as a biodegradable drug carrier. The method involves treatment of a cellulose source with a mixture of phosphoric acid and nitric acid and a small but adequate amount of sodium nitrite at room temperature for a period until the desired level of oxidation is achieved. The present invention also provides a method whereby cellulose materials such as cotton linters, purified cotton papers, xcex1-cellulose, purified wood pulp, microcrystalline cellulose, powdered cellulose, or like materials can be readily oxidized to produce products that can be used to develop a sustained- and/or controlled release delivery systems for drugs and other chemical agents. The oxidized cellulose of this invention is also suitable to stop bleeding during surgery, prevent the formation and reformation of post surgical adhesions, promote bone regeneration, and for use in periodontal therapy.